Abstract
A multifaceted desktop and field based geotechnical survey and geohazard mapping project was completed for over 25,000 ha of mixed rural and urban land identified for potential future urban growth and development within the Kaipara District in Northland, New Zealand. The project included desktop reconnaissance, field mapping, interpretation and presentation of data.
An initial high-level desktop study was completed to identify areas of focus within each of the eight “Indicative Growth Areas” (IGAs). The desktop study utilised multiple methods to develop preliminary hazard maps and identify focus areas within the IGAs.
Once the focus areas had been identified, the field mapping program was carried out to ground-truth the preliminary desktop maps, and to identify geomorphological features and geotechnical hazard potential. The field mapping program utilised land-based mapping and photography, as well as drone photography and videography where possible.
The data collected from the desktop and field studies was then used to determine risk ratings for various hazards within the IGAs. ArcGIS was then used to produce various layers which were collated to generate combined geohazard maps for IGA.
1. Introduction
The importance of robust desktop reconnaissance tools prior to embarking on a field mapping exercise for large areas of land was illustrated during a mapping project for over 25,000 ha of mixed rural and urban zoned land identified for potential future urban growth within the Kaipara District (KDC) in Northland. The project commenced with a high-level desktop study identifying focus points across eight “Indicative Growth Areas” (IGAs) in the region, which were defined by Kaipara District Council.
Stereoscopic aerial photographs, published geology maps, historical aerial photographs, borehole records from the New Zealand Geotechnical Database, Google Earth and in-house GIS were all used during the desktop study to identify focus areas within the IGAs. The field investigation concentrated on identification of geomorphological features and potential geohazards within the IGAs. Land-based mapping and photography, as well as specifically targeted oblique angle drone photography was used to gain variable points of view of features and areas of focus. GIS was used to produce maps showing slope angle, inferred slope stability, liquefaction potential, consolidation settlement potential, acid sulphate soil potential and onsite effluent disposal potential, resulting in combined geohazard maps for the study areas. A robust desktop study utilising various tools and information sources allows for enhanced efficiencies and focus during the field mapping exercise. This holistic approach to mapping projects consequently allows for enhanced data and risk interpretation which, when presented appropriately, will assist in informing policy, planning and development.
2. Desktop Study
As given above, the first step was to complete a high-level desktop assessment utilising a range of methods to assess each of the IGAs and identify focus areas for the field mapping phase. The various methods for the desktop study were undertaken collaboratively within the project team to produce preliminary hazard maps so that field geologists were able to spend more time assessing features and areas of interest during the limited available time in the field. The various desktop study tools are summarised as follows.
2.1 Geological Maps
The geological settings of IGAs were established through a review of the Institute of Geological and Nuclear Sciences (GNS) 1:250,000 map (Edbrooke and Brook 2009), and supplemented by a site walkover to observe the landform and outcrops, where accessible. Although the QMAP series are a widely accepted account of the surface expression of geological units across the country, at a regional 1:250,000 scale, the detail and accuracy of unit boundaries and structural features are indicative only. Although Northland Regional Council 1:100,000 soil and rock type maps (Sutherland et al 1980, Crippen and Markham 1981) are available for some of the IGAs, the QMAP was considered more suited to assist with identification of hazard focus areas across the IGAs due to their focus on the broader geological setting rather than high detail soil and rock settings
The IGAs were underlain by many different geological formations including Tauranga Group Alluvium, Kariotahi Group Dunes, Kerikeri Group Volcanics, Waitemata Group sedimentary formations, Northland Allochthon and Whangarei Limestone; all of which exhibited variable characteristics.
The geological maps (example from of the Omamari IGA is shown in Figure 1) were integral to the desktop study in that they provided a base map and framework to identify various features that are synonymous with different geology types, such as accepted stable slope angles, low lying alluvial areas which may be prone to liquefaction and/or formations that are prone to instability.
2.2 Aerial Photography
Historical aerial photographs from Retrolens New Zealand, stereo-paired aerial photos and Google Earth dating back to 1940 were reviewed as part of the desktop study. The photographs were viewed under the context of identifying general changes to the landform and development within the IGAs, and to refine the geological mapping based on the topography. Aerial photographs were useful for studying large areas, however were limited by a lack of georeferencing within the Retrolens historical aerial photographs and the physical stereopairs. It was therefore sometimes difficult to locate and focus in on target features.
For the stereoscopy, a mid-range scale of 1:25,000 was selected to provide project coverage. Images were assessed to identify geomorphic features such as headscarps, hummocky and irregular-shaped landscapes, displaced blocks and debris lobes that may be indicative of recent or historic landslide activity. Approximate extents of alluvium and colluvium deposits in hillside gullies and valley areas were also mapped, as they are more likely to be susceptible to liquefaction and consolidation settlement.
Google Earth allowed for interactive observations and assessments of the wider IGAs. By utilising a combination of exaggerated elevation, three-dimensional terrain and oblique angled navigation it was possible to identify and target areas with potential geohazards and geomorphological features to focus on during the field study.
2.3 Survey Data
LIDAR and survey maps were used to form the base for many of the GIS map outputs and allowed for widespread slope modelling.
2.4 Borehole Logs
Borehole data from the KDC groundwater bore database and NZGD were used to identify representative standing groundwater depths across the IGAs.
The datasets were often limited due to lack of publicly available boreholes. This is likely due to the lower density of development in the Northland region compared to other more populated regions in New Zealand. It also reflects the fact that the NZGD is a relatively young tool and a large amount of historical information will not therefore be publicly available.
3. Field Study
A review of the available data was undertaken to identify key areas within each of the IGAs where additional, site-specific data collection would be targeted. The field study phase generally consisted of geomorphological mapping on the ground in those areas where access was possible and use of the drone for more inaccessible areas.
3.1 Land-Based Mapping
The mapping was not intended to provide a detailed geomorphic map of the area, but to ground-truth the preliminary desktop maps and to observe geomorphic features that could not easily be interpreted from aerial photographs. Geohazard data was gathered at various scales from areas such as road cuttings (Figure 2 – Left), elevated viewpoints (Figure 2 – Right), dune cliff faces, low-lying alluvial plains, rolling farm land and estuarine areas.
3.2 Drone Photography
The use of drone photography and videography facilitated alternative points of view of focus areas and allowed for high level observations of features in wider geographical settings (Figure 3). This allowed for observations of areas that were unable to be accessed by land, as well as providing high resolution oblique aerial images which assist in refining the nature and extent of geomorphic features. However, drone surveys were often hindered by high winds in coastal and exposed areas.
4. Reporting and GIS Outputs
The data gathered from the desktop and field studies, along with soil and slope parameters from published sources and previous experience with the present geological formations, was analysed and collated to form conclusions on groundwater, active faults, slope angles, slope instability, liquefaction, consolidation, volcanic hazards, acid sulphates, on-site effluent disposal and on-site stormwater disposal.
In order to quantify the geotechnical hazard potential for land planning, a broad framework based on a three-level hazard profile was developed. This system defined potential hazard areas as Low, Medium and High, relative to the level of impact they may potentially have on future development. Risk ratings for various hazards were generally determined from published data, previous experience in the region and field observations. Hazard risk assessments included:
- Published slope data (primarily from GNS), previous studies in the region, our experience with slope stability assessments in similar geology, our field observations of stable slope angles and instability features for slope instability risk (as shown in Figure 4);
- Soil/rock type (e.g. likely presence of compressible organic soils), age of formation, and topographical setting for consolidation settlement risk;
- Age of geological formation, topographical setting, groundwater depth and soil/rock type (e.g. clay soils vs. loose sands) for liquefaction potential;
- Published data (Acid Sulphate Soil Risk maps (Opus 2017) provided by KDC), Soil/rock type (e.g. likely presence of organic soils) and topographical setting (e.g. low lying areas that may have been influenced by seawater) for acid sulphate soil risk; and
- Soil/rock type (e.g. likely soil permeability), topography and groundwater depth for on-site effluent disposal potential maps.
Due to the limited coverage of LiDAR data over the study area, the LINZ Topo50 20 m contours (vertical accuracy ≤ 10 m) were used to create digital elevation models, and then slope models of the IGAs (As shown in Figure 5).
The assessed risk ratings for each hazard were then used as inputs for Arc GIS to produce various maps for each of the IGAs, including maps for:
- IGA Vicinity showing the IGA regional location and boundary;
- Geology showing the geological setting and formations within the IGA (Figure 1);
- Geomorphological Interpretation showing the mapped areas identified as susceptible to geohazards during the field mapping;
- Slope classification showing the slope angles across the IGA (Figure 5);
- Slope instability profile showing the assessed instability risk across the IGAs;
- Settlement susceptibility showing areas of low to high consolidation settlement risk;
- Liquefaction hazard showing areas assessed to have risk of seismically induced liquefaction;
- Acid sulphate soil risk showing areas with mapped geology types considered to have potential risk of acid sulphate influence;
- On-site effluent disposal potential maps showing areas assessed to be potentially suitable for on-site disposal and those that are unlikely to be suitable.
Combined geohazard maps based on a summation of the primary geotechnical constraints considered for each IGA was then produced (Example for the Dargaville IGA shown in Figure 6). These combined Geohazard maps depicted the minimum risk level shown.
5. Conclusion
The importance of a multi-faceted approach to mapping was illustrated during a geotechnical survey and geohazard mapping project which covered approximately 25,000 ha over eight IGAs in Northland, New Zealand.
A robust desktop study was carried out which utilised a variety of tools to formulate a high-level understanding of the geological setting, geomorphological features and potential geohazards within the IGAs, which allowed for focus areas to be identified and preliminary hazard maps to be produced prior to embarking on the field aspect of the investigation. Publicly available aerial photographs (both recent and historical) played an important part in the desktop study as they allowed for a high-level assessment of the wider IGAs and identification of focus areas, with opportunity to review the development of specific features over time.
The field study placed significant reliance upon the findings of the desktop study and focussing on the target areas identified and assessing changes to landform and associated geomorphological features/geohazards.
The source and detail/limitations of available data was a key consideration for the project. The small scale of the available geological maps and aerial photographs meant risk area boundaries were considered approximate only. It should also be noted that the majority of contour data was from LIDAR surveys which can sometimes be very low resolution in remote areas. The significance of utilising multiple tools to study the IGAs and back-checking earlier assumptions against results of various assessment methods became apparent throughout the project lifecycle.
6. Acknowledgements
Thank you to the extraordinarily supportive and encouraging team at ENGEO for their assistance in completing this project, preparing this paper and sponsorship to attend this conference.
Thank you also to the Kaipara District Council for approval to report on this exciting project and for having the foresight to invest in gaining a deeper understanding of the characteristics of their region.
Thank you to the New Zealand geotechnical community for providing the tools and support for young geotechnical professionals to better their knowledge and training.
7. References
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ENGEO Limited, 2019. Geotechnical Assessment Reports – Kaipara District. 15601.000.000 – 15601.000.006.
Edbrooke, S. W., Brook, F. J., 2009. Geology of the Whangarei Area – 1:250000 Geological Map 2. Institute of Geological & Nuclear Sciences.
Institute of Geological & Nuclear Sciences., 2004. A Review of Natural Hazards Information for Northland Region.
Opus International Consultants Limited., 2017. Joint Council Submission, Acid Sulphate Soils – Northland. 1-13807.00-Draft.
